code: def bw(self):
      for src in self.sources:
          src.grad = None
      self.out.backward(self.out, retain_graph=True)
  
file_path: xformers/xformers/benchmarks/benchmark_indexing.py
imports: import random
  import torch
  from utils import DTYPE2STR, benchmark_main_helper2, product_dict
  import xformers.ops
label_desc: Use image node_id 3 for calling the IndexSelectBenchmark obj's underlying member method code with example usage: obj.bw() without return types